The invention relates to the field of oil burner control systems for furnaces and more particularly to electronic burner control circuits having fail-safe capabilities.
In the prior art systems, specifically represented by Lenski U.S. Pat. No. 3,770,365, and application of Lenski entitled, "Burner Control," Ser. No. 362,387 filed on May 21, 1973, and issued as U.S. Pat. No. 3,829,276 on August 13, 1974 both assigned to the assignee of this application, utilize an electromechanical safety swith in a secondary portion of a control circuit for the purpose of shutting down a burner motor when, after a call for heat by a thermostat, a flame is not detected at the burner within certain time limits. Because the safety switch in these prior systems utilizes a thermal switch for timing, it was not always possible to obtain or predict as precisely as desired, the amount of time that would elapse from a call for heat to the deenergization of the burner motor and igniter in the event a flame was not detected. This uncertainty in timing results from a number of factors including the fact that thermal type switches are particularly sensitive to variations in both line voltages and ambient temperatures, especially for time delays of 10 seconds or more.
In addition, failure of the triac device, controlling the motor and igniter, in a shorted or closed condition, would prevent the burner from switching off when there is no longer a call for heat or permit the furnace to fill with fuel after the burner flame has failed.